1. Field of the Invention
The present invention relates to a feedback control apparatus. More particularly, the present invention relates to a Proportional-integral-Derivative (PID) controller for controlling a control value based on the output value of a control object to be controlled.
2. Description of the Related Art
A portable electronic device with a camera module is equipped with a hand-trembling compensation apparatus. The hand-trembling compensation apparatus typically includes a Voice Coil Motor (VCM), a driver using a piezo-electric actuator, and a feedback controller for controlling the driver based on the output value of the driver.
The driver has hysteresis according to non-linear, time-varying driving direction. Generally, the driver is controllable in a very narrow range and has a rapid phase variation in a control range, and its operational characteristics vary with a change in an operational environment such as temperature.
Conventionally, the non-linear driver is controlled through an approximation to a linear function using a PID controller or a lead-lag controller or by a non-linear system control algorithm taking a large computation volume, such as neural network control or fuzzy control. The use of linear control technology is not effective in sufficiently compensating for the non-linearity. For example, a driver that moves an image sensor on a driving surface by means of a VCM may experience a drag phenomenon as the driving range becomes narrower with respect to the same driving command due to friction on the driving surface. Although the drag can be resolved by applying a regular high-frequency vibration signal to the driver, the resulting increased noise and power consumption makes this method unsuitable for portable electronic devices. The use of non-linear control technology, such as neural network control or fuzzy control requires a large amount of computation. Hence, this approach does not achieve a sufficient control performance when a low-spec microcontroller suitable for a small-size electronic device is used.
PID control is the most widespread control method due to its simple control mechanism and easy implementation. Despite the advantage of satisfactory control performance for a linear system, a general PID controller has limited performance because a real system has complex characteristics including non-linearity and time variability. An adaptive control scheme like self-tuning PID control has been proposed to solve this problem. However, the adaptive control scheme is based on the linearity of a driver and requires strict operational conditions. The fundamental cause of the problem is that a complex system is controlled based on a mathematic model.
A fuzzy control scheme has complex system characteristics including non-linearity and time variability. That's why fuzzy control is used for system control that does not allow for mathematical modeling. A fuzzy controller does not require mathematical modeling and provides control by language rules, e.g., “IF-THEN”, adaptively according to instantaneous system situations. Therefore, the fuzzy control scheme is popular and flexible in dealing with non-linear control issues. A typical fuzzy controller forms control rules by receiving an error and its derivative as inputs. This input mechanism is similar to that of the PID controller, However, compared to the PIP controller, the fuzzy controller is complex to implement and takes a large amount of computation, and the control rules are difficult to design.
As described above, the conventional PID control or fuzzy control requires an accurate mathematic model or has the shortcomings of a large computation volume and implementation complexity. Therefore, it is not suitable for fast and high-precision control using a low-performance microcontroller.
Accordingly, there exists a need for a feedback controller that can provide for fast control of a non-linear driver in real time with high precision by means of a small-sized circuit structure using a low-spec microprocessor.